Conventionally, a three-way catalyst (catalytic converter) is provided in an exhaust passage of a multi-cylinder internal combustion engine in order to purify an exhaust gas discharged from the engine. As is well known, the three-way catalyst has an “oxygen storage function (oxygen storage and release function)” to store and release oxygen in accordance with components flowing into the three-way catalyst. Hereinafter, the three-way catalyst is also simply referred to as a “catalyst”, the gas flowing into the catalyst is also referred to as a catalyst inflow gas. Further, the gas flowing out from the catalyst is also referred to as a catalyst outflow gas.
One (conventional apparatus) of the conventional air-fuel ratio control apparatuses comprises an upstream air-fuel ratio sensor disposed in the exhaust passage of the engine at a position upstream of the catalyst, and a downstream air-fuel ratio sensor disposed in the exhaust passage of the engine at a position downstream of the catalyst. The conventional apparatus obtains, based on an amount of an air introduced into the engine, a “base fuel injection amount to have an air-fuel ratio of a mixture supplied to the engine (air-fuel ratio of the engine) coincide with the stoichiometric air-fuel ratio”, and performs a feedback-correction on the base fuel injection amount based on an output value of the upstream air-fuel ratio sensor and an output value of the downstream air-fuel ratio sensor (see, for example, Patent Literature No. 1). Further, the conventional apparatus changes a correction amount of the feedback-correction based on a period (time duration) in which the output value of the downstream air-fuel ratio sensor changes from a value corresponding to a rich air-fuel ratio (or a lean air-fuel ratio) with respect to the stoichiometric air-fuel ratio to a value corresponding to a lean air-fuel ratio (or a rich air-fuel ratio) with respect to the stoichiometric air-fuel ratio (that is, the time duration according to an inversion period, and thus, corresponding to a variation/parameter which varies in accordance with an oxygen storage capacity of the catalyst). Accordingly, the conventional apparatus can perform an air-fuel ratio control in accordance with the oxygen storage capacity of the catalyst.